Established electrodynamic sound transducer concepts usually use diaphragms which are centrally connected to an electromagnetic voice coil and are caused to vibrate either by Lorentz forces induced by current flow or by air movements. Depending on the operating mode, either an electric current is converted into mechanical movement or a diaphragm movement is converted into an electric current. With these design forms, for example, moving coil loudspeakers or microphones are obtainable, which today are characterized by high sound intensity and natural sound reproduction.
However, the suspension of the diaphragm and the special electrodynamic coupling with the aid of a magnet result in certain overall depths and vibration characteristics which are not suitable for every design and application situation. For this reason, sound transducers which are not based on a diaphragm-coil-magnet combination, but which use piezoelectric effects for sound conversion, were developed in the last few decades. These sound transducers include electroactive ceramics or plastic materials and allow direct sound conversion due to the changes of the macroscopic sound transducer dimensions as a function of an electric field. For example, the application of a voltage to a piezoelectric diaphragm or foil results in a change of the longitudinal extension (d31) and thickness extension (d33). The d31 effect in particular results in bending of the layer, which may be effectively used for sound radiation. Conversely, a mechanical load results in an electric charge transfer within the layer, which may generally be used for sound detection. These piezoelectric sound transducers require only very minor deflections. In the case of loudspeakers, the deflections are typically in the range of several hundred μm, while the deflections in the range of microphone applications are only several hundred μm to just a few nm or pm. In the case of loudspeakers as well as in the case of microphones, the deflections are very strongly dependent on the frequency. At higher frequencies, smaller deflections occur than at lower frequencies. As a result of these basic conditions, in particular foil transducers having only a very small distance from other surfaces may be implemented, which previously were not accessible with the established electromagnetic diaphragm-coil systems.
One possible specific embodiment for unusual transducer geometries is known, for example, from U.S. Pat. No. 4,638,207 A, which describes the use of a piezoelectric polymer which is based on polyvinylidene fluoride (PVDF) for manufacturing balloon-shaped loudspeakers. PVDF strips, which are embedded between an outer and an inner coating, are applied to a balloon, or the balloon itself is formed from such strips.
In one further embodiment, DE 10 2010 043 108 A1 describes a piezoelectric sound transducer using a piezoelectric plastic material. In particular, a sound transducer is described which is essentially composed of a carrier layer and a layer of a piezoelectric plastic carrier applied thereto, the piezoelectric plastic layer not entirely covering the carrier layer, but having recesses.
The design of preferably effective and reliable piezoelectric transducers, however, previously required a compromise in the attachment of the transducer foils. Full-surface, direct fixation of the transducer foils to surfaces, for example by adhesive bonding, creates a secure attachment of the transducer, but results in a very strong impairment of the deflectability, which disadvantageously affects the transducer effectiveness. For this reason, the electroactive foils are situated as a composite on a flexible carrier foil, and the composite is held mechanically at its edges, to obtain a preferably unimpaired vibration behavior. This enables designs in which the composite is sufficiently mechanically fixed and therefore stabilized and may otherwise vibrate freely. This results in a favorable radiation or reception characteristic; however, it reduces the overall mechanical load capacity of the transducer at the non-attached locations and is thus disadvantageous.
It is therefore the object of the present invention to provide an electroactive sound transducer foil which exhibits good vibration behavior and which, due to its design and geometry, may be easily and securely attached to a wide variety of surface geometries, without the vibration characteristics of the foil being too strongly impaired.